The present invention relates to photovoltaic cells and fabrication methods thereof. More specifically, the present invention relates to organic synthesis processes for fabricating semiconductor nanoparticles for use in high-efficiency photovoltaic cells.
Photovoltaics generate electrical power by converting solar energy into direct current electricity through the use of semiconductors that exhibit the photovoltaic effect. The photovoltaic effect is the generation of current or electric charge in a material upon exposure to light. Photovoltaics are best known, and most commonly employed, as a method of generating electric power through the use of solar cells to convert solar energy into usable electric power.
Photovoltaic (PV) cells, commonly referred to as solar cells, use a variety of photovoltaic materials. Materials presently used include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide (CIGS). Generally, CIGS cells are not limited to the use of copper, indium, gallium, and selenium, and can use semiconducting elements from Groups I, III, and VI of the periodic table because these elements exhibit high absorption coefficients and versatile optical and electrical characteristics, allowing devices made of such materials to be tuned and manipulated depending on the needs of the product.
Photovoltaic cells are very small products, fabricated by layering different chemicals on top of each other in very small amounts using different chemical. As such, the fabrication process is sequential, and must be performed very precisely for the cell to be effective. Photovoltaic materials are contained in an absorber layer, where light is absorbed and photons excite electrons in atoms of the photovoltaic material. This interaction moves the electrons from the valence band of the atom to the conduction band, wherein the electron is energized enough that it is free from binding with its atom and may move freely between atomic particles. Such delocalized electron movement provides electric current in metals and other conducting materials and may be harnessed as power. When the excited electron leaves its valence band, a “hole” is created in this valence band where an electron once was. This electron hole represents the absence of an object, and can be thought of as a negative object or a single positive charge which may also move between atoms as electrons move to fill holes in other atoms. The electron-hole pair and the relationship between the two objects is crucial to the function of photovoltaic cells.
A CIGS photovoltaic cells typically includes several layers of different materials to produce the desired solar-to-electrical energy conversion. Such a CIGS cell 101 is schematically shown in FIG. 1. The top layer of the cell 101 is an antireflection (AR) coating 111, which inhibits solar energy from being reflected off the cell surface and becoming unusable. Front contacts 110 (two of which are shown) are overlaid by the AR coating 111 and provide a negative terminal for electron flow in the cell 101. Below the contacts 110 and AR coating 111 is a transparent conducting oxide (TCO) layer (“window”) 109 and a metal oxide buffer layer 108. Below this, a bottom buffer layer 107, possibly composed of an amorphous n-type material such as cadmium sulfide (CdS), zinc sulfide (ZnS) or tin sulfide (SnS) having a higher bandgap than the absorber layer 106, facilitates the travel of electrons to the contacts 110, as well as inhibits the reverse of this electron flow. Below the buffer layer 107 is a CIGS absorber layer 106, where the photovoltaic effect occurs and where electrons and holes are generated. The electrons move towards the front contacts 110, while the holes move through an ohmic junction 105 toward a back contact layer 104 formed of a conductive material, for example, molybdenum, tungsten, titanium, or copper. The back contact layer 104 may also reflect unabsorbed light back into the absorber layer 106. A pair of boundary layers 103 are represented as being disposed on opposite surfaces of what is generally referred to herein as a substrate 102 on which the other layers of the cell 101 are deposited. The boundary layers 103 inhibit the substrate 102 from becoming electrically charged and decreasing cell efficiency or even short-circuiting connections, as well as protects the substrate 102 from the environment. The substrate layer 102 provides the structural foundation for the cell 101, and is commonly composed of a glass material, though polyimide, metal foils, and other structural materials may be used to improve weight and flexibility.
The fabrication of CIGS cells typically begins with the substrate layer 102, and additional layers are deposited on top of each other through various chemical processes. The basic fabrication processes for CIGS cells are well known to those skilled in the art. However, many of these processes are inefficient and costly, or they may produce inferior cells in an attempt to improve process efficiency. It would be advantageous to provide a fabrication process for CIGS cells or cell components which produced higher-efficiency cells and cell components without a dramatic corresponding increase in fabrication cost.